An emulsion is a system typically having a liquid dispersed in an immiscible liquid. Dividing a sample of liquefied emulsion by mass requires transferring, by pumping for example, the emulsion from its container to an empty container located on an electronic balance or other similar balance or measuring device. This transfer operation can take several minutes to complete, during which time the emulsion may not be properly agitated. If an emulsion sits idle, or is without proper agitation during long transfers, heavy particles in the emulsion may settle. Settling destroys the homogeneity of the emulsion system causing erroneous measurements in coating experiments.
One method for avoiding erroneous readings due to lack of proper agitation is temporarily removing the container from the balance during the transfer operation and during mixing so that the emulsion is on the balance for a final weight reading. This method is not very practical because weighings of the emulsion throughout the transfer operation are often needed and desired more often than not. These readings are very useful for providing feedback for an automated delivery system. It is therefore impractical to move the tared container back and forth between the balance and a nearby mixer. Ideally, an emulsion is continuously stirred or mixed as long as it sits on the balance. Mixing should be interrupted to allow the balance to stabilize for intermediate weighings and resumed afterwards to minimize settling.
Unfortunately, most mixers used on or with a balance can add a source of unnecessary vibrational noise to the electronic balance readings. A propeller-type stirrer, such as disclosed in U.S. Pat. No. 4,725,149, can be cantilevered over the balance (i.e. positioned over the container) to effectively mix emulsions when the emulsions are not being weighed. Because the prop touches the fluid during weighings, the prop will transfer vibrational noise to the balance even when it is stopped. If the prop is temporarily raised above the container during weighings to prevent touching, a second problem arises. The second problem is that small amounts of the sample to be weighed are retained on the prop which significantly degrades the weighing results.
Another mixing method uses a stirring mechanism which stays with the container before, during and after the weighings. Magnetic stirrers are one example of such a mechanism. Typically, magnetic motors mounted under a container drive a magnetic stir bar placed inside the container by creating a rotating magnetic field that penetrates the container. Magnetic stirrers are ideal for most laboratory mixing applications. They are inexpensive and easily cleaned and usually stay with the sample until it is dumped. Unfortunately, a magnetic stirrer mechanism usually requires some sort of connection to an external power source. Whether driven by air, water or electricity, a tube or wire connected to an external power source is usually required. This connection, regardless of how light-weight or how carefully supported, adversely affects the stability and accuracy of precision balances.
U.S. Pat. No. 4,725,149 discloses an automatic dissolving device for dyes utilizing an electronic balance and a magnet-type stirring device mounted on the balance. U.S. Pat. Nos. 3,744,764; 3,211,433 and 2,466,468 disclose stirring devices that are fluid driven. U.S. Pat. No. 4,991,973 discloses a device for simultaneously agitating and weighing a sample wherein an electronic balance has a magnetic stirring means mounted which enables the agitating and weighing operations to be performed simultaneously and by the same device. In each of these prior art devices, however, the stirring means is integral to the balance; that is, the mixer cannot be easily removed and transported without making the balance inoperative. Accordingly, it will be appreciated that it would be highly desirable to have a simple apparatus for weighing and stirring a sample wherein the mixer is simply connected to the balance.
It is desirable to have a small, light-weight mixer that does not use interfering wires or tubing. Battery operated stirrers are a consideration but are not practical for precision balances because precision balances often lack the range to support the weight of a battery and electromagnet.